Hi @Mundo23 

Use a Notification Email Script to evaluate conditions and override the subject line using email.setSubject().

Ref: Email Client Script Dynamically Changing Subject Line 

Step 1: Create the Notification Email Script

• Navigate to System Policy > Email > Notification Email Scripts in your ServiceNow instance.
• Click New.
• Set the Name (like  set_dynamic_client_subject).
• In the Script field, add your logic.

Sample code :

if (current.client == 'Client A') {

    email.setSubject("Important Update for Client A: " + current.number);

} else if (current.client == 'Client B') {

    email.setSubject("Important Update for Client B");

} else {

    email.setSubject("General Notification: " + current.short_description);

}

Step 2: Add the Script to your Notification

• Open the Email Notification you want to send.
• Call email script from within the body : ${mail_script:set_dynamic_client_subject}